A touch sensitive device offers a simple, intuitive interface to a computer or other data processing device. Rather than using a keyboard to type in data, a user can transfer information by touching an icon or by writing or drawing on a touch sensitive panel. Touch panels are used in a variety of information processing applications. Interactive visual displays often include some form of touch sensitive panel. Integrating touch sensitive panels with visual displays is becoming more common with the emergence of next generation portable multimedia devices such as cellphones, personal data assistants (PDAs), and handheld or laptop computers.
Various methods have been used to determine touch location, including capacitive, resistive, acoustic and infrared techniques. One touch detection technology, referred to as Surface Acoustic Wave (SAW), uses high frequency waves propagating on the surface of a glass screen. Attenuation of the waves resulting from contact of a finger with the glass screen surface is used to detect touch location. SAW employs a “time-of-flight” technique, where the time for the disturbance to reach the pickup sensors is used to detect the touch location. Such an approach is possible when the medium behaves in a non-dispersive manner, such that the velocity of the waves does not vary significantly over the frequency range of interest.
Another type of touch detection technology involves sensing certain vibrations created by a touch in the bulk material of the touch sensitive substrate. These vibrations are denoted bending waves and may be detected using sensors typically placed on the edges of the substrate. Signals generated by the sensors are analyzed to determine the touch location. Bending wave touch technology is particularly useful for applications where the touch panel may become dirty or scratched because such interferences do not significantly degrade touch location performance.
In many applications, such as signature capture, or electronic tablets, accuracy in determining the location of the touch is an important factor. Further, there is a need to enhance both accuracy and speed of touch location determination in small devices, such as handheld units. The present invention addresses these and other needs.